It would help if they discussed brain volume as determinative of human categorization.
The problem with brain volume is that it’s only one attribute amongst many that seems to constantly vary with these proposed ancestors of man. If you look at all the key traits together, there’s no clear progression at all. So, if you arrange your hereditary tree based on brain volume, then examine another trait, such as the tooth arrangement, or ability to walk upright, you will find that your tree is out of order with regards to those other traits.